1. The present invention relates to a developer composition for lithographic printing plates.
2. Description of the Prior Art
Photo-crosslinkable, photosensitive polymer compositions have already been put widely into practical use as resists for photo-etching. This type of photosensitive polymers and polymer compositions are superior in various properties such as sensitivity, storage stability, image strength, sensitizing ability and the like, compared with diazo photosensitive materials which had previously been put into practice as lithographic printing plates and the like. For this reason, the photo-crosslinkable photosensitive polymers have widely used in lithographic printing plates in recent years. When these photosensitive polymers are exposed to actinic radiation, they are crosslinked and hardened so that the exposed polymer is relatively insoluble as compared with the unexposed polymer. Such difference in properties between the exposed area and unexposed area can be utilized to make printing plates photographically. For the purpose a lithographic printing plate base is coated with a photosensitive polymer along with other additives, for example, sensitizer and/or coloring agent to form a coated layer and the coated plate is then exposed imagewise to radiation to form soluble and insoluble areas. The unhardened soluble polymer in the unexposed area is then removed for the purpose of development, for example, by treating the plate with a developer solvent or solution capable of dissolving the unexposed polymer. The developer solution which remains on the surface of the plate and which contains the unexposed, unhardened soluble polymer is removed from the surface of the plate by rinsing the plate with water or treating it with an aqueous desensitizer whereby the plate is cleaned. Thus, the plate has an image formed thereon by a hydrophilic area not receiving printing ink and a lipophilic area receiving printing ink and it is used in printing operation.
Various developer solutions including developer compositions comprising .gamma.-butyrolactone and an acid are known in the art. However, the prior art developer solutions are not always satisfactory with respect to lifetime in repeated use, stability of developing effect to variation in developing conditions such as developing time, storage stability and the like, although they can well develop a lithographic printing plate as long as they are fresh. In addition, if the developer solution used has a poor affinity to water, water rinsing or treatment with an aqueous desensitizer subsequent to development cannot afford efficient removal of the remaining developer solution from the surface of the plate, which may result in scumming or image unevenness. Moreover, if the viscosity of the developer solution is too high, particularly in mechanical development by means of an automatic developing machine, the rate of developer solution fed through the spray nozzles is decreased due to decreased circulation efficiency of the developer solution. As a result, the developer solution is not sufficiently supplied throughout the lithographic printing plate, which may cause an uneven development. More specifically, the mechanical developing process of lithographic printing plates is generally performed by feeding a processing solution such as a developer solution onto the surface of the lithographic printing plate through spray nozzles which are arranged in the direction perpendicular to the conveying direction of the plate and which are composed of small openings having a diameter not exceeding several millimeters. Therefore, if the viscosity of the developing solution is too high, it is difficult to spray the solution efficiently through the spray nozzles and the developer solution cannot be fed in a uniform rate onto the surface of the printing plate. In a conventional mechanical developing process, the developer solution is usually passed through a filter which is provided in the course of the circulation line of the developing solution in order to entrap the polymer removed from the plate. In such filtration, if the viscosity of the developer solution is too high, the flow rate of the developer solution is so decreased that it cannot be supplied sufficiently, resulting in an uneven development.
Therefore, there is a continuing need for providing a developer composition for lithographic printing plates having a coated layer of a photosensitive polymer, said composition being free from the above-mentioned disadvantages of the prior art developer solutions.